Optical device with large thermal impedance

ABSTRACT

Embodiments of an optical device, an array of optical devices, and a technique for fabricating the optical device or the array are described. This optical device is implemented on a substrate (such as silicon), and includes a thermally tunable optical waveguide that has good thermal isolation from its surroundings. In particular, a portion of a semiconductor in the optical device, which includes the optical waveguide, is free standing above a gap between the semiconductor layer and the substrate. By reducing the thermal coupling between the optical waveguide and the external environment, the optical device can be thermally tuned with significantly less power consumption.

RELATED APPLICATIONS

This application is a continuation of, and hereby claims priority under35 U.S.C. §120 to, pending U.S. patent application Ser. No. 12/415,858,entitled “Optical Device with Large Thermal Impedance,” by inventorsJohn E. Cunningham, Ashok V. Krishnamoorthy, Ivan Shubin, GuoLiang Liand Xuezhe Zheng, filed on 31 Mar. 2009.

This application is related to pending U.S. patent application Ser. No.12/415,882, entitled “Thermal Tuning of an Optical Device,” by GuoLiangLi, Ashok V. Krishnamoorthy, John E. Cunningham, Ivan Shubin and XuezheZheng, filed on 31 Mar. 2009, the contents of which are hereinincorporated by reference. This application is also related to pendingU.S. patent application Ser. No. 12/415,886, entitled “Dual-LayerThermally Tuned Optical Device,” by GuoLiang Li, John E. Cunningham,Ashok V. Krishnamoorthy, Ivan Shubin and Xuezhe Zheng, filed on 31 Mar.2009, the contents of which are herein incorporated by reference.

GOVERNMENT LICENSE RIGHTS

This invention was made with Government Support under Agreement No.HR0011-08-9-0001 awarded by DARPA. The Government has certain rights inthe invention.

BACKGROUND

1. Field of the Invention

The present invention relates to techniques for communicating opticalsignals. More specifically, the present invention relates to an opticaldevice with a thermally tunable optical waveguide.

2. Related Art

Wavelength division multiplexing (WDM) is widely used to communicatemodulated data at different carrier wavelengths on a common opticalwaveguide. WDM can overcome optical-fiber congestion, which is apotential problem in optical modules that include parallel opticaltransceivers with one channel per optical fiber. In particular, bysignificantly reducing the number of optical fibers per optical module,WDM multiplexing can simplify optical modules, thereby reducing theircost and size.

In dense WDM (DWDM), a narrow spacing between adjacent wavelengths isused. This is typically achieved by modulating data directly onto ahighly stable optical carrier, and then combining multiple carriers inan optical fiber. DWDM allows a large number of channels to beaccommodated within a given wavelength band, and thus offers highperformance.

In DWDM, a variety of optical devices are used as: modulators,multiplexers (such as add filters), de-multiplexers (such as dropfilters), filters and switches. In order to compensate for fabricationvariation, temperature variation and/or laser wavelength drift, theseoptical devices are typically phase-tuned to a particular wavelength fora given channel. Depending on the system requirements, a tuning range ofat least 180° may be needed.

Thermal tuning is a popular tuning technique because it provides theability to produce large phase shifts. Existing thermal tuningtechniques include direct heating (which is implemented by doping in anoptical waveguide) and indirect heating (in which a heater is proximateto the optical waveguide). Typically, the direct-heating technique ismore energy-efficient than indirect heating, but it can prevent theoptical waveguide from performing additional functions (because of theconstraint on the doping density), and it can introduce additionaloptical losses due to free-carrier absorption (which can degrade thequality factor of an optical resonator).

In principle, optical devices can be made on silicon substrates, becausesilicon provides many benefits for optical communication. For example,the high index-of-refraction contrast between silicon and silicondioxide can be used to create sub-micron waveguides to confine lightwith spatial densities that are up to 100× larger than in a single-modeoptical fiber. Furthermore, by using a silicon-on-insulator (SOI)technology, a silicon waveguide can be surrounded by silicon dioxide onall four sides, which facilitates low-loss, on-chip waveguides andactive devices (such as detectors and modulators). Silicon-based opticaldevices can be used to implement a wide variety of optical componentsfor use in WDM communication. These silicon-based optical devices offernumerous advantages, including: miniaturization, low-energy modulation,the ability to integrate with other devices in silicon, and/or theability to leverage the large, existing silicon manufacturinginfrastructure.

Unfortunately, there are problems associated with silicon-based opticaldevices. A notable problem is the high thermal conductivity of silicon.While this helps remove the heat dissipated by electrical circuits, itcan make it more difficult to thermally tune a silicon-based opticaldevice. In particular, because the operating wavelength of asilicon-based optical device (such as the resonant wavelength of anoptical resonator) strongly depends on temperature, the operatingwavelength is typically tuned using either direct or indirect heating tochange the operating temperature of the silicon-based optical device.However, the high thermal conductivity of silicon results in excessivethermal coupling to the surrounding environment. Consequently, thermaltuning of silicon-based optical devices often consumes adisproportionately large amount of energy (typically, 50-100 mW for aphase shift of 180°). This high power consumption can offset theadvantages provided by silicon, and makes it more difficult to usesilicon-based optical devices to implement optical communication (suchas WDM) in computing systems (especially in systems that have multipleinstances of the optical devices).

Hence, what is needed is an optical device that can be thermally tunedwithout the above-described problems.

SUMMARY

One embodiment of the present invention provides an optical device thatincludes: a substrate, an intermediate layer deposited on the substrate,and a semiconductor layer deposited on the intermediate layer. Thissemiconductor layer includes a thermally tunable optical waveguide andone or more vias that extend from a top of the semiconductor layer to abottom of the semiconductor layer which is proximate to the intermediatelayer. A portion of the semiconductor layer is free standing above a gapbetween the semiconductor layer and the substrate, where the gapcorresponds to a removed sacrificial portion of the intermediate layer,which was accessed through the one or more vias. This gap increasesthermal impedance between the semiconductor layer and the substrate,thereby reducing power consumption associated with thermal tuning of theoptical waveguide. Moreover, another portion of the semiconductor layeris supported by a remaining portion of the intermediate layer.

Note that the substrate may include silicon, the intermediate layer mayinclude an oxide (such as silicon dioxide) and/or the semiconductorlayer may include silicon. For example, the substrate, the intermediatelayer and the semiconductor layer may comprise a silicon-on-insulator(SOI) technology.

In some embodiments, air is at least partially removed from the gap.

Furthermore, the optical device may include a heater defined in thesemiconductor layer (which may be in or proximate to the opticalwaveguide). For example, the heater may include a passive resistor or anactive device. This heater may be used to thermally tune the opticalwaveguide. Note that power consumption by the heater may be less than10-20 mW for a 180° phase shift in the optical waveguide.

Additionally, the optical device may include one or more electricalcouplings between the semiconductor layer and one or more power-supplycontacts through one or more bridges over the gap. However, in otherembodiments, the one or more electrical couplings between thesemiconductor layer and the one or more power-supply contacts occur inthe remaining portion of the intermediate layer.

In some embodiments, the optical waveguide is included in: an opticalfilter, an optical multiplexer and/or an optical de-multiplexer.

Another embodiment provides an array with multiple instances of theoptical device. In this embodiment, the gap in a given optical deviceincreases thermal impedance to other instances of the optical device.Moreover, the heater in the given optical device may facilitateprogramming the temperature in the given optical device, which can bedifferent from the temperature in one or more other instances of theoptical device.

Another embodiment provides a method for fabricating an optical device.During this method, a thickness of a semiconductor layer in a tri-layerstructure, which includes a substrate, an intermediate layer depositedon the substrate, and a semiconductor layer deposited on theintermediate layer, is reduced. Then, one or more vias that extend froma top of the semiconductor layer to a bottom of the semiconductor layer,which is proximate to the intermediate layer, are opened. Next, asacrificial portion of the intermediate layer is removed so that aportion of the semiconductor layer is free standing above a gap betweenthe semiconductor layer and the substrate. Note that another portion ofthe semiconductor layer is supported by a remaining portion of theintermediate layer. Moreover, a thermally tunable optical waveguide inthe semiconductor layer, which is substantially positioned above thegap, is defined, thereby reducing power consumption associated withthermal tuning of the optical waveguide.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a block diagram illustrating a tunable optical device inaccordance with an embodiment of the present invention.

FIG. 2 is a block diagram illustrating a tunable optical device inaccordance with an embodiment of the present invention.

FIG. 3A is a block diagram illustrating a tunable optical device inaccordance with an embodiment of the present invention.

FIG. 3B is a block diagram illustrating a tunable optical device inaccordance with an embodiment of the present invention.

FIG. 4 is a block diagram illustrating a tunable optical device inaccordance with an embodiment of the present invention.

FIG. 5 is a block diagram illustrating a tunable optical device inaccordance with an embodiment of the present invention.

FIG. 6A is a block diagram illustrating a tunable optical device inaccordance with an embodiment of the present invention.

FIG. 6B is a block diagram illustrating a tunable optical device inaccordance with an embodiment of the present invention.

FIG. 7A is a block diagram illustrating a tunable optical device inaccordance with an embodiment of the present invention.

FIG. 7B is a block diagram illustrating a tunable optical device inaccordance with an embodiment of the present invention.

FIG. 8 is a block diagram illustrating a tunable optical device inaccordance with an embodiment of the present invention.

FIG. 9 is a flow chart illustrating a process for fabricating an opticaldevice in accordance with an embodiment of the present invention.

Note that like reference numerals refer to corresponding partsthroughout the drawings.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the invention, and is provided in the context ofa particular application and its requirements. Various modifications tothe disclosed embodiments will be readily apparent to those skilled inthe art, and the general principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the present invention. Thus, the present invention is notintended to be limited to the embodiments shown, but is to be accordedthe widest scope consistent with the principles and features disclosedherein.

Embodiments of an optical device, an array of optical devices, and atechnique for fabricating the optical device or the array are described.This optical device is implemented on a substrate (such as silicon), andincludes a thermally tunable optical waveguide with a high thermalresistance to the surrounding external environment and a low thermalresistance to a localized heater. In particular, the thermal resistancesassociated with thermal dissipation paths from a heater in the opticaldevice to an external environment via electrodes and via the substrateare increased, while the thermal resistance between the opticalwaveguide and the heater is decreased.

By reducing the thermal coupling between the optical waveguide and theexternal environment, the optical device can be thermally tuned withsignificantly less power consumption than existing silicon-based opticaldevices (for example, 10-100× less power consumption). This reduction inpower consumption may facilitate systems that use DWDM with a channelspacing of approximately 50 GHz, as well as high-performance computingsystems that include optical interconnects. Moreover, in these systems,the optical device may: reduce the number of components, reduce thecost, and/or increase reliability.

We now describe embodiments of an optical device that includes anoptical waveguide. Thermal tuning of an optical waveguide, which isimplemented on silicon, is often based on the increase in the index ofrefraction of silicon with temperature. By changing the temperature inthe optical waveguide, the index of refraction, as well as the opticalphase (which is proportional to the index of refraction) of thepropagating laser light in the optical waveguide, can be tuned. In orderto achieve efficient thermal tuning, the temperature change in theoptical waveguide needs to be maximized for the available heater power.

FIG. 1 presents a block diagram illustrating a cross-sectional view of atunable optical device 100. This optical device includes: a substrate110, an intermediate layer 112 deposited on the substrate 110, and asemiconductor layer 114 deposited on the intermediate layer 112. Thissemiconductor layer includes a thermally tunable resonant opticalwaveguide, as illustrated by p/n junction 116. (As shown in FIG. 2, theoptical waveguide may be implemented in a ring-modulator configuration,so the two portions of p/n junction 116 shown in FIG. 1 are actuallypart of one continuous structure.) A resonant wavelength of this opticalwaveguide is a function of the temperature of p/n junction 116. Thisoptical waveguide may be included in a variety of optical components,such as: an optical filter, an optical multiplexer, an opticalde-multiplexer and an optical add/drop.

In optical device 100, intermediate layer 112 is partially etched toproduce one or more gaps 118 between the semiconductor layer 114 and thesubstrate 110. For example, the one or more gaps 118 may be fabricatedby removing a sacrificial portion of the intermediate layer 112. Thissacrificial portion may be removed using a chemical-etch process thataccesses the intermediate layer 112 through one or more vias 122 thatextend from a top of the semiconductor layer 114 to a bottom of thesemiconductor layer 114 which is proximate to the intermediate layer112.

Consequently, a portion of the semiconductor layer 114 is undercut andis free standing above the one or more gaps 118, and a central portionof the semiconductor layer 114 is supported by a remaining, centralportion of the intermediate layer 112. Note that the one or more gaps118 reduce thermal coupling between the semiconductor layer 114 and thesubstrate 110. If a heater 124 is implemented in the optical waveguideor proximate to the optical waveguide on the free-standing portion ofthe semiconductor layer 114, the power consumption associated withthermal tuning of the optical waveguide is reduced because the thermalimpedance of the one or more gaps 118 is larger (10× or more) than thethermal impedance of the intermediate layer 112 (which allows a greatertemperature difference to be applied and maintained across thisinterface). For example, power consumption by the heater may be lessthan 20 mW for a 180° phase shift in the optical waveguide (which is atleast 2.5-5× lower than existing optical devices).

In an exemplary embodiment, the substrate 110 includes silicon, theintermediate layer 112 includes an oxide (such as silicon dioxide),and/or the semiconductor layer 114 includes silicon. Therefore, thesubstrate 110, the intermediate layer 112 and the semiconductor layer114 may comprise a silicon-on-insulator (SOI) technology. Furthermore,the silicon semiconductor layer 114 may be 0.5 μm thick and thesilicon-dioxide intermediate layer may have a thickness between 0.001and 10 μm. In addition, the sidewalls of the one or more gaps 118 may bethermally oxidized to avoid scattering losses.

Note that air in the one or more gaps 118 has a much lower thermalconductivity (approximately 0.02 W/m-K) compared to silicon dioxide (1.4W/m-K), which can reduce the heat dissipation to substrate 110 by afactor of 10×. In some embodiments, air is at least partially removedfrom the one or more gaps 118 to further increase the thermal impedance.For example, a soft-vacuum may be achieved using a getter, and bysubsequently sealing the one or more vias 122.

Electrical connections to the heater 124 and/or the optical waveguidemay be implemented using one or more optional air-bridge electricalcouplings 120 between the central portion of the semiconductor layer 114and one or more power-supply contacts (which are shown in FIGS. 2 and3). However, in other embodiments, the one or more electricalconnections between the central portion of the semiconductor layer 114and the one or more power-supply contacts may occur in the remaining,central portion of the intermediate layer 112. For example, heater 124(such as a passive resistor or an active device) may be situated in thisremaining, central portion of the intermediate layer 112. Thisnon-undercut area has a low thermal impedance to the optical waveguide,and a high impedance to the external environment of optical device 100.

In some embodiments, resistive heating is implemented inside the opticalwaveguide (instead of using a heater, such as heater 124, which is inclose proximity to the optical waveguide). For example, one or moreheating elements may be integrated in the optical waveguide and acontrolled amount of current may be sourced from a tuning circuit tothese heating elements. However, doping the optical waveguide toimplement the one or more heating elements may increase optical lossesdue to free-carrier absorption, which can degrade the quality factor ofa resonator device.

Note that optical device 100 may include multiple instances of thestructure shown in FIG. 1. For example, there may be an array of opticalwaveguides implemented in the semiconductor layer 114, each of which isat least partially thermally isolated from the substrate 110 by gaps(such as the one or more gaps 118). In this case, these gaps will alsoreduce thermal coupling between the optical waveguides in the array,thereby reducing or eliminating thermal cross-talk.

Moreover, the temperatures of the optical waveguides in the array may beindividually programmed, so that one or more optical waveguides can havedifferent temperatures than other optical waveguides in the array.Therefore, in an array of optical waveguides (or more generally, opticalcomponents), heaters and/or heating elements may be used to producearbitrary, programmable thermal profiles across the array or to createthe necessary tuning, modulation and/or other functionality. Once theheaters or heating elements are turned on, the power to each may bechosen to increase the temperature of each resonant device by theappropriate amount to achieve an operating temperature in the desiredrange. Moreover, the temperature control mechanism may include biasedtemperature control.

In some operating conditions, the absolute wavelength range of the arraymay experience drift. This can occur if the ambient temperature changesor if there is some global change in the temperature of the substrate110. However, even in this situation, active thermal management of theresonant wavelengths of the optical waveguides will still produce andmaintain the correct wavelengths as long as the chip does not experiencethermal runaway. This possibility can be substantially mitigated ifactive cooling is also used to control and maintain the globaltemperature of the substrate 110 at a pre-determined level.

FIG. 2 presents a block diagram illustrating a top view of a tunableoptical device 200, which includes optical waveguides 210. Opticalwaveguide 210-1 is situated above a gap (shown by the hatched region)and is supported by a remaining portion 214 of the intermediate layer112 (FIG. 1). Note that the intermediate layer 112 (FIG. 1) in thisembodiment is etched on three of four sides of the remaining portion214.

Operating bias for optical waveguide 210-1 may be provided by highlydoped, low-resistance electrical modulator contacts 212 via air-bridgeelectrical couplings, such as optional electrical couplings 120 (FIG.1). Note that these modulator contacts can also be used to conduct heatand/or current from a chip that includes one or more waveguides toassociated laser elements.

A variety of configurations may be used for the one or more vias 122(FIG. 1), which facilitate partial etching of the intermediate layer 112(FIG. 1). In some embodiments, the vias (or openings) are implemented inthe substrate 110 (FIG. 1). FIG. 3A presents a block diagramillustrating a bottom view of a tunable optical device 300 with aninternal opening 310, and FIG. 3B presents a block diagram illustratinga bottom view of a tunable optical device 350 with an external opening360. These openings may be fabricated by dry etching a silicon substrateto a thickness between 0.1 and 0.5 μm. Then, the openings may be createdby removing the silicon to expose a portion of the intermediate layer112 (FIG. 1).

Internal opening 310 (FIG. 3A) may provide a more symmetric undercut,and may avoid a discontinuity in the optical waveguide (cladding/nocladding), which may reduce optical losses. However, external opening360 may offer a larger thermal impedance between the substrate 110(FIG. 1) and the semiconductor layer 114 (FIG. 1). Both of the openingconfigurations may allow electrical connections to be routed without airbridges.

In optical devices, such as optical device 100 (FIG. 1), there aretypically three heat dissipation paths: a first path dissipates heatthrough the active semiconductor layer 114 (FIG. 1) to the electricalconnections; a second path dissipates heat through the intermediatelayer 112 (FIG. 1) to the substrate 110 (FIG. 1); and a third pathdissipates heat through a top layer above the semiconductor layer 114(FIG. 1) to the air (which is typically negligible). Note that thetemperature change in the optical waveguide is proportional to thethermal impedance of all of the heat dissipation paths. Because thethird path is typically negligible, in order to maximize the temperaturechange, the thermal impedances of the first and second paths need to beincreased.

Optical device 100 (FIG. 1) provided a technique for increasing thethermal impedance of the second path. We now describe additionaltechniques for increasing the thermal impedance of the first and thesecond paths, which may be used separately and/or in conjunction witheach other, as well as with the technique shown in FIG. 1. By combiningall of these techniques, the thermal impedance between the opticalwaveguide in the optical device and the external environment may beincreased by over an order of magnitude. This can reduce the powerconsumed during tuning of the optical waveguide by an order ofmagnitude. In addition, optical losses in the optical waveguide due tofree-carrier absorption can also be reduced by more than 4×.Consequently, these techniques can significantly reduce the power budgetfor silicon-based optical devices in WDM systems and computing systemsthat include optical interconnects.

FIG. 4 presents a block diagram illustrating a cross-sectional view of atunable optical device 400. This optical device includes: a substrate410, an intermediate layer 412 deposited on the substrate 410, asemiconductor layer (which includes a thermally tunable opticalwaveguide 414) deposited on the intermediate layer 412, and a top layer416 deposited on the semiconductor layer. This top layer includes one ormore vias from an upper surface of the top layer 416 to a bottom surfaceof the top layer 416, which is proximate to the semiconductor layer.Moreover, optical device 400 includes electrodes 418, deposited on thetop layer 416 and in the one or more vias, which electrically couple tooptical waveguide 414 in the semiconductor layer through contacts 420.

A thermal impedance of the first path may be increased by using amaterial in portions of the electrodes 418-2 and 418-4 which has a lowerthermal conductivity (i.e., a higher thermal impedance) than a remainderof the electrodes 418-1 and 418-3, thereby reducing the powerconsumption associated with thermal tuning of the optical waveguide 414by reducing thermal coupling from an external environment to the opticalwaveguide 414. For example, portions of the electrodes 418-2 and 418-4may include indium-tin-oxide (ITO), or another material with a highelectrical conductivity and a low thermal conductivity. ITO is highlyconductive, optically transparent, and has a very low thermalconductivity (less than 10 W/m-K, compared with 163 W/m-K for siliconand 401 W/m-K for copper). This can potentially increase the thermalimpedance in the first path more than 10×.

Note that the intermediate layer 412 and the top layer 416 may beelectrical insulators and dielectric materials that have a low opticalindex of refraction and a low thermal conductivity. Moreover, thesubstrate 410, the intermediate layer 412 and the semiconductor layermay comprise a SO/technology.

Additionally, the semiconductor layer in a central region of the opticalwaveguide 414 may be doped (for example, using implantation) to define aheater that facilitates thermal tuning of the optical waveguide 414.(Alternatively, a separate heater, such as heater 124 in FIG. 1, may beused.) Power consumption by the heater may be less than 10-20 mW for a180° phase shift in the optical waveguide.

In some embodiments, doping of the central region of the semiconductorlayer in the optical waveguide 414 (such as a region defined by theapproximate lateral extent of an evanescent wave) may be less than thedoping of the semiconductor layer in lateral regions 426 surrounding theoptical waveguide 414, thereby reducing optical losses in the opticalwaveguide 414 and reducing heat dissipation along the second path. Forexample, doping in bottom-center region 422 and top-center region 424may be 10¹⁷/cm³ and doping in the lateral regions 426 may be 10¹⁸/cm³.However, in some embodiments, the bottom-center region 422 (but not thetop-center region 424) of the optical waveguide 414 is doped, therebyfurther reducing optical losses in the optical waveguide 414. Note thatthe central regions of optical waveguide 414 may be 0.5 μm wide and thelateral regions 426 may 2 μm wide.

As shown in FIG. 5, which presents a block diagram illustrating across-sectional view of tunable optical device 500, in some embodimentsa thickness 510-2 of the intermediate layer 412 is thicker under theoptical waveguide 414 than a thickness 510-1 of lateral regions of theintermediate layer 412. By increasing the thickness, the thermalimpedance of the second path may be increased. Note that the thicknessmay be increased by implanting oxygen atoms into the silicon substrate410 underneath the optical waveguide 414. Then, a layer of the substrate410 may be thermally oxidized to increase the thickness 510-2.

FIG. 6A presents a block diagram illustrating a top view of a tunableoptical device 600. In this optical device, optical waveguide 414 (whichincludes a central rib 610 having width 626, and a total width 624) isthermally coupled to electrodes 612 by multiple regions 614 in thesemiconductor layer which have a higher electrical conductivity thanadditional regions 616 that are interdigited with the regions 614. Inmaterials other than indium-tin oxide, this higher electricalconductivity is often accompanied by higher thermal conductivity. Toaddress this, in some embodiments the electrode material(s) and/orshape(s) are selected to reduce the thermal conductivity withoutsignificantly increasing the electrode resistance.

Regions 614 may have a symmetry axis or a principal axis (through thecenter of mass) that is approximately perpendicular (for example, within15° of perpendicular) to a symmetry axis or a principal axis (throughthe center of mass) of the optical waveguide 414 in a plane of thesemiconductor layer. While regions 614 are illustrated with rectangularregions, in other embodiments a variety of shapes, such as zig-zag linesand/or non-symmetric shapes may be used.

Regions 614 in the semiconductor layer may have a higher doping thanregions 616 and the optical waveguide 414. In an exemplary embodiment,regions 614 have a 10× larger doping, such as 10²⁰/cm³. Moreover, awidth 618 of the regions 614 may be at least five times smaller than aperiod 622 of the regions 614. In an exemplary embodiment, regions 614have a thickness of 0.1 μm, a width 618 of 1 μm, and a length 620between 2 and 3 μm. Furthermore, there may be a spacing of 9 μm betweenadjacent regions 614. This configuration may increase the thermalresistance in the first path by an order of magnitude, and it mayincrease the thermal resistance in the second path as well (due toreduced area for downward heat conduction).

As shown in FIG. 6B, which presents a block diagram illustrating a topview of a tunable optical device 650, in some embodiments, the doping ofthe semiconductor layer in the optical waveguide 414 alternates betweena first type of doping region 660-1 and a second type of doping region660-2, where the doping in the first type of doping region 660-1 ishigher than in the second type of doping region 660-2. For example, thefirst type of doping region 660-1 may be doped and the second type ofdoping region 660-2 may be un-doped. This alternative doping profilereduces optical losses in the optical waveguide, and may reduce heatdissipation along the second path. In addition, central rib 610 may beun-doped or lightly doped.

In a variation on the preceding embodiments, the heater may beimplemented in a separate layer from the optical waveguide 414, forexample, using a dual SOI technology. This is shown in FIG. 7A, whichpresents a block diagram illustrating a cross-sectional view of atunable optical device 700. In this device, a heater (such as heater 124in FIG. 1) is implemented in semiconductor layer 710 (such as silicon),which is separated from the optical waveguide 414 by intermediate layer712. (Alternatively, semiconductor layer 710 may be a conductive polymeror a dielectric, such as silicon nitride.) For example, the heater maybe implemented by doping semiconductor layer 710 to 10¹⁸/cm³ underneaththe optical waveguide 414 to 10¹⁸/cm³. Note that intermediate layer 712may be an electrical insulator which is 0.5 μm thick, and which is adielectric material with low thermal conductivity (such as silicondioxide).

This spatial separation of functionality may combine the advantages ofdirect heating and indirect heating. In particular, optical waveguide414 and intermediate layer 712 in optical device 700 may provideefficient optical confinement with reduced free-carrier optical losses.In addition, additional functions may be implemented in the opticalwaveguide 414. Note that heat transfer between the heater insemiconductor layer 710 and the optical waveguide 414 may occur viaelectrodes 418, which is very efficient. For example, heat transfer maybe mediated by a portion of electrodes 418 in vias in intermediate layer712. Moreover, heat transfer between the heater in semiconductor layer710 and the substrate 410 may be reduced by intermediate layer 412.

Furthermore, a width 714 of the heater in semiconductor layer 710 may bewider than a width 716 of the optical waveguide 414. This geometricconfiguration may facilitate heat transfer between the heater and theoptical waveguide 414. Consequently, power consumption by the heater inoptical device 700 may be less than 10-20 mW for a 180° phase shift inthe optical waveguide 414.

Heat transfer to the substrate 410 may be further reduced byundercutting intermediate layer 412 (using the technique shown in FIG.1). This is shown in FIG. 7B, which presents a block diagramillustrating a cross-sectional view of a tunable optical device 750. Inparticular, there may be gaps 760 in the intermediate layer 412underneath at least a portion of the heater in semiconductor layer 710.Note that the undercut etching can be performed after the ohmic contactsare formed and intermediate layer 712 is protected by surrounding metalwalls.

In some embodiments, the vertical order of semiconductor layer 710 andthe optical waveguide 414 are reversed. This is shown in FIG. 8, whichpresents a block diagram illustrating a cross-sectional view of atunable optical device 800. In these embodiments, semiconductor layer710 may include poly-silicon or amorphous silicon.

Optical devices 100 (FIG. 1), 200 (FIG. 2), 300 (FIG. 3A), 350 (FIG.3B), 400 (FIG. 4), 500 (FIG. 5), 600 (FIG. 6), 700 (FIG. 7A), 750 (FIG.7B) and/or 800 may include fewer components or additional components.Moreover, although these optical devices are illustrated as having anumber of discrete items, FIGS. 1-8 are intended to be functionaldescriptions of the various features that may be present rather thanstructural schematics of the embodiments described herein. Consequently,in these embodiments, two or more components may be combined into asingle component, and/or a position of one or more components may bechanged.

A wide variety of fabrication techniques may be used to fabricate theoptical devices in FIGS. 1-8, as known to one of skill in the art. Inaddition, while the preceding embodiments illustrate single instances ofan optical device, any of the preceding embodiments may be implementedin an array that includes multiple instances.

Note that embodiments of the optical device can also be used when theoptical waveguide is electrically tuned. In these embodiments, it isstill important that the optical waveguide be thermally isolated fromits external environment.

One or more of these optical devices may be included in a multi-chipmodule (MCM) (such as a switch or a processor) and/or in a system thatinclude the MCM. This MCM may include an array of chip modules (CMs) orsingle-chip modules (SCMs), and a given SCM may include at least onesemiconductor die. Note that the MCM is sometimes referred to as a‘macro-chip.’ Furthermore, the semiconductor die may communicate withother semiconductor dies, CMs, SCMs, and/or devices in the MCM usingproximity communication of electromagnetically coupled signals (which isreferred to as ‘electromagnetic proximity communication’), such ascapacitively coupled signals and/or proximity communication of opticalsignals (which are, respectively, referred to as ‘electrical proximitycommunication’ and ‘optical proximity communication’). In someembodiments, the electromagnetic proximity communication includesinductively coupled signals and/or conductively coupled signals.

Embodiments of the MCM may be used in a variety of applications,including: VLSI circuits, telephony, storage area networks, datacenters, networks (such as local area networks), and/or computer systems(such as multiple-core processor computer systems). For example, the MCMmay be included in a backplane that is coupled to multiple processorblades, or the MCM may couple different types of components (such asprocessors, memory, I/O devices, and/or peripheral devices). In someembodiments, the MCM performs the functions of: a switch, a hub, abridge, and/or a router.

Note that the computer systems may include, but are not limited to: aserver, a laptop computer, a personal computer, a work station, amainframe computer, a blade, an enterprise computer, a data center, aportable-computing device, a supercomputer, a network-attached-storage(NAS) system, a storage-area-network (SAN) system, and/or anotherelectronic computing device. Moreover, note that a given computer systemmay be at one location or may be distributed over multiple,geographically dispersed locations.

We now describe embodiments of a process for fabricating an opticaldevice, such as optical device 100 (FIG. 1). FIG. 9 presents a flowchart illustrating a process 900 for fabricating an optical device.During this process, a thickness of a semiconductor layer in a tri-layerstructure, which includes a substrate, an intermediate layer depositedon the substrate, and a semiconductor layer deposited on theintermediate layer, is reduced (910). Then, one or more vias that extendfrom a top of the semiconductor layer to a bottom of the semiconductorlayer, which is proximate to the intermediate layer, are opened (912).Next, a sacrificial portion of the intermediate layer is removed so thata portion of the semiconductor layer is free standing above a gapbetween the semiconductor layer and the substrate (914). Note thatanother portion of the semiconductor layer is supported by a remainingportion of the intermediate layer. Moreover, a thermally tunable opticalwaveguide in the semiconductor layer, which is substantially positionedabove the gap, is defined (916), thereby reducing power consumptionassociated with thermal tuning of the optical waveguide.

In some embodiments of process 900, there may be additional or feweroperations. Moreover, the order of the operations may be changed, and/ortwo or more operations may be combined into a single operation.

The foregoing descriptions of embodiments of the present invention havebeen presented for purposes of illustration and description only. Theyare not intended to be exhaustive or to limit the present invention tothe forms disclosed. Accordingly, many modifications and variations willbe apparent to practitioners skilled in the art. Additionally, the abovedisclosure is not intended to limit the present invention. The scope ofthe present invention is defined by the appended claims.

What is claimed is:
 1. An optical device, comprising: a substrate; anintermediate layer deposited on the substrate; and a semiconductorlayer, deposited on the intermediate layer, that includes a thermallytunable optical waveguide and a via that extends from a top of thesemiconductor layer to a bottom of the semiconductor layer which isproximate to the intermediate layer; wherein a portion of thesemiconductor layer is free standing above a gap between thesemiconductor layer and the substrate, wherein the gap corresponds to aremoved sacrificial portion of the intermediate layer which isaccessible through the via, and wherein air is at least partiallyremoved from the gap; wherein another portion of the semiconductor layeris supported by a remaining portion of the intermediate layer; andwherein the gap increases thermal impedance between the semiconductorlayer and the substrate, thereby reducing power consumption associatedwith thermal tuning of the optical waveguide.
 2. The optical device ofclaim 1, wherein the substrate includes silicon.
 3. The optical deviceof claim 1, wherein the intermediate layer includes an oxide.
 4. Theoptical device of claim 3, wherein the oxide includes silicon dioxide.5. The optical device of claim 1, wherein the substrate, theintermediate layer and the semiconductor layer comprise asilicon-on-insulator technology.
 6. The optical device of claim 1,further comprising a heater defined in the semiconductor layer.
 7. Theoptical device of claim 6, wherein the heater includes a passiveresistor or an active device.
 8. The optical device of claim 1, furthercomprising an electrical coupling between the semiconductor layer and apower-supply contact through a bridge over the gap.
 9. The opticaldevice of claim 1, wherein an electrical coupling between thesemiconductor layer and a power-supply contact occurs in the remainingportion of the intermediate layer.
 10. The optical device of claim 1,further comprising a heater, which is in or proximate to the opticalwaveguide, and which is used to thermally tune the optical waveguide,wherein power consumption by the heater is less than 20 mW for a 180°phase shift in the optical waveguide.
 11. The optical device of claim 1,wherein the optical waveguide is included in: an optical filter, anoptical multiplexer or an optical de-multiplexer.
 12. The optical deviceof claim 1, further comprising a seal positioned in the via, wherein theseal is configured to seal the gap to prevent entry of air into the gapthrough the via.
 13. An array of multiple instances of an opticaldevice, wherein a given instance of the optical device comprises: asubstrate; an intermediate layer deposited on the substrate; and asemiconductor layer, deposited on the intermediate layer, that includesa thermally tunable optical waveguide and a via that extends from a topof the semiconductor layer to a bottom of the semiconductor layer whichis proximate to the intermediate layer; wherein a portion of thesemiconductor layer is free standing above a gap between thesemiconductor layer and the substrate, wherein the gap corresponds to aremoved sacrificial portion of the intermediate layer which isaccessible through the via, and wherein air is at least partiallyremoved from the gap; wherein another portion of the semiconductor layeris supported by a remaining portion of the intermediate layer; andwherein the gap increases thermal impedance between the semiconductorlayer and the substrate, thereby reducing power consumption associatedwith thermal tuning of the optical waveguide.
 14. The array of opticaldevices of claim 13, wherein the gap increases thermal impedance toother instances of the optical device.
 15. The array of optical devicesof claim 13, wherein the substrate, the intermediate layer and thesemiconductor layer comprise a silicon-on-insulator technology.
 16. Thearray of optical devices of claim 13, further comprising a heaterdefined in the semiconductor layer.
 17. The array of optical devices ofclaim 16, wherein the heater facilitates a programmable temperature inthe given instance of the optical device, which can be different fromthe temperature in one or more other instances of the optical device.18. The array of optical devices of claim 13, further comprising aheater, which is in or proximate to the optical waveguide, and which isused to thermally tune the optical waveguide, wherein power consumptionby the heater is less than 20 mW for a 180° phase shift in the opticalwaveguide.
 19. A method for fabricating an optical device, comprising:reducing a thickness of a semiconductor layer in a tri-layer structure,which includes a substrate, an intermediate layer deposited on thesubstrate, and a semiconductor layer deposited on the intermediatelayer; opening a via that extends from a top of the semiconductor layerto a bottom of the semiconductor layer which is proximate to theintermediate layer; removing a sacrificial portion of the intermediatelayer so that a portion of the semiconductor layer is free standingabove a gap between the semiconductor layer and the substrate, whereinanother portion of the semiconductor layer is supported by a remainingportion of the intermediate layer; at least partially removing air fromthe gap; and defining a thermally tunable optical waveguide in thesemiconductor layer, which is substantially positioned above the gap,thereby reducing power consumption associated with thermal tuning of theoptical waveguide.
 20. The method of claim 19, wherein the methodfurther comprises sealing the via to prevent entry of air into the gapthrough the via.